1. Field of the Invention
The present invention relates generally to electroluminescent (EL) panels, and more specifically, to a circuit for driving an EL panel to achieve low noise operation.
2. Background of the Invention
Electroluminescent (EL) panels are in common use as backlights for keyboards and displays. Recent uses of EL panels include wrist watches, cellular telephone displays and keyboards, notebook computers and personal digital assistants (PDAs). In order to produce illumination from an EL panel, an alternating high voltage power supply is required. An EL panel driver includes a high voltage power supply, and a mechanism for switching the high voltage power supply output to produce a high voltage output of alternating polarity for connection to the EL panel.
EL panels are fabricated as a laminate structure, with a phosphor-dielectric layer sandwiched between a pair of electrodes. Outer cover layers are then applied to the outer surfaces of the electrodes, producing a insulated and protected panel. When the EL panel is driven with the above-described alternating high voltage signal, vibration is produced via a piezoelectric effect between the electrodes. Since typical drive frequencies are in the audible range, the vibration is perceivable to the user of an EL panel-equipped device as audible noise.
Techniques have been adopted to reduce the EL panel noise, such as current limiting the discharge of the EL lamp and approximating a sinusoidal drive waveform, but.these techniques leave residual audible noise that may be perceived by an enduser. Additionally, the circuitry required for implementing the above-described techniques is complex and requires die area and additional power for operation.
EL panels also degrade over their lifetime due to ionic migration within the phosphor-dielectric layer. Decreasing frequency of the drive signal extends the phosphor life, but an EL panel will still become degraded and lose brightness over time. Brightness is sometimes adjustable in devices that incorporate EL panels, but a manual adjustment required by the user is less desirable than an automatic compensation for aging of the panel. The brightness of EL panels is often not adjustable, so the brightness of an EL panel in a typical installation decreases over time.
Therefore, it would be desirable to provide a apparatus and method for driving EL panels that will reduce panel noise. It would further be desirable to provide and apparatus and method for driving EL panels that will compensate for panel aging effects.
The above objective of providing a low noise apparatus and method for driving EL panels is accomplished in a circuit for driving EL panels and method for driving EL panels. The circuit includes an output for connection to the EL panel that is coupled to an output circuit for alternating an output of a high voltage power supply. The output circuit is driven by a waveform generator to provide an output alternating frequency and a waveshaping for shaping the alternating voltage at the output in a first-order exponential shape. A ratio of a full period of the waveform generator to a time constant of the exponential shape is within the range of 3 to 8.
The above objective of compensating for loss of EL panel brightness due to aging is achieved in a circuit for driving EL panels and method for driving EL panels that sets the above-described time constant in conformity with the capacitance of the EL panel, so that as capacitance of the EL panel decreases as the EL panel ages, the drive voltage supplied to the EL panel increases.
The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings.